Network Working Group JP. Vasseur, Ed.
Request for Comments: 5440 Cisco Systems
Category: Standards Track JL. Le Roux, Ed.
France Telecom
March 2009
Path Computation Element (PCE) Communication Protocol (PCEP)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
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Vasseur & Le Roux Standards Track [Page 1]
RFC 5440 PCEP March 2009
Abstract
This document specifies the Path Computation Element (PCE)
Communication Protocol (PCEP) for communications between a Path
Computation Client (PCC) and a PCE, or between two PCEs. Such
interactions include path computation requests and path computation
replies as well as notifications of specific states related to the
use of a PCE in the context of Multiprotocol Label Switching (MPLS)
and Generalized MPLS (GMPLS) Traffic Engineering. PCEP is designed
to be flexible and extensible so as to easily allow for the addition
of further messages and objects, should further requirements be
expressed in the future.
Vasseur & Le Roux Standards Track [Page 2]
RFC 5440 PCEP March 2009
Table of Contents
1. Introduction ....................................................5
1.1. Requirements Language ......................................5
2. Terminology .....................................................5
3. Assumptions .....................................................6
4. Architectural Protocol Overview (Model) .........................7
4.1. Problem ....................................................7
4.2. Architectural Protocol Overview ............................7
4.2.1. Initialization Phase ................................8
4.2.2. Session Keepalive ...................................9
4.2.3. Path Computation Request Sent by a PCC to a PCE ....10
4.2.4. Path Computation Reply Sent by The PCE to a PCC ....11
4.2.5. Notification .......................................12
4.2.6. Error ..............................................14
4.2.7. Termination of the PCEP Session ....................14
4.2.8. Intermittent versus Permanent PCEP Session .........15
5. Transport Protocol .............................................15
6. PCEP Messages ..................................................15
6.1. Common Header .............................................16
6.2. Open Message ..............................................16
6.3. Keepalive Message .........................................18
6.4. Path Computation Request (PCReq) Message ..................19
6.5. Path Computation Reply (PCRep) Message ....................20
6.6. Notification (PCNtf) Message ..............................21
6.7. Error (PCErr) Message .....................................22
6.8. Close Message .............................................23
6.9. Reception of Unknown Messages .............................23
7. Object Formats .................................................23
7.1. PCEP TLV Format ...........................................24
7.2. Common Object Header ......................................24
7.3. OPEN Object ...............................................25
7.4. RP Object .................................................27
7.4.1. Object Definition ..................................27
7.4.2. Handling of the RP Object ..........................30
7.5. NO-PATH Object ............................................31
7.6. END-POINTS Object .........................................34
7.7. BANDWIDTH Object ..........................................35
7.8. METRIC Object .............................................36
7.9. Explicit Route Object .....................................39
7.10. Reported Route Object ....................................39
7.11. LSPA Object ..............................................40
7.12. Include Route Object .....................................42
7.13. SVEC Object ..............................................42
7.13.1. Notion of Dependent and Synchronized Path
Computation Requests ..............................42
7.13.2. SVEC Object .......................................44
7.13.3. Handling of the SVEC Object .......................45
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RFC 5440 PCEP March 2009
7.14. NOTIFICATION Object ......................................46
7.15. PCEP-ERROR Object ........................................49
7.16. LOAD-BALANCING Object ....................................54
7.17. CLOSE Object .............................................55
8. Manageability Considerations ...................................56
8.1. Control of Function and Policy ............................56
8.2. Information and Data Models ...............................57
8.3. Liveness Detection and Monitoring .........................57
8.4. Verifying Correct Operation ...............................58
8.5. Requirements on Other Protocols and Functional
Components ................................................58
8.6. Impact on Network Operation ...............................58
9. IANA Considerations ............................................59
9.1. TCP Port ..................................................59
9.2. PCEP Messages .............................................59
9.3. PCEP Object ...............................................59
9.4. PCEP Message Common Header ................................61
9.5. Open Object Flag Field ....................................61
9.6. RP Object .................................................61
9.7. NO-PATH Object Flag Field .................................62
9.8. METRIC Object .............................................63
9.9. LSPA Object Flag Field ....................................63
9.10. SVEC Object Flag Field ...................................64
9.11. NOTIFICATION Object ......................................64
9.12. PCEP-ERROR Object ........................................65
9.13. LOAD-BALANCING Object Flag Field .........................67
9.14. CLOSE Object .............................................67
9.15. PCEP TLV Type Indicators .................................68
9.16. NO-PATH-VECTOR TLV .......................................68
10. Security Considerations .......................................69
10.1. Vulnerability ............................................69
10.2. TCP Security Techniques ..................................70
10.3. PCEP Authentication and Integrity ........................70
10.4. PCEP Privacy .............................................71
10.5. Key Configuration and Exchange ...........................71
10.6. Access Policy ............................................73
10.7. Protection against Denial-of-Service Attacks .............73
10.7.1. Protection against TCP DoS Attacks ................73
10.7.2. Request Input Shaping/Policing ....................74
11. Acknowledgments ...............................................75
12. References ....................................................75
12.1. Normative References .....................................75
12.2. Informative References ...................................76
Appendix A. PCEP Finite State Machine (FSM) ......................79
Appendix B. PCEP Variables .......................................85
Appendix C. Contributors .........................................86
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RFC 5440 PCEP March 2009
1. Introduction
[RFC4655] describes the motivations and architecture for a Path
Computation Element (PCE) based model for the computation of
Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS)
Traffic Engineering Label Switched Paths (TE LSPs). The model allows
for the separation of PCE from Path Computation Client (PCC), and
allows for the cooperation between PCEs. This necessitates a
communication protocol between PCC and PCE, and between PCEs.
[RFC4657] states the generic requirements for such a protocol
including that the same protocol be used between PCC and PCE, and
between PCEs. Additional application-specific requirements (for
scenarios such as inter-area, inter-AS, etc.) are not included in
[RFC4657], but there is a requirement that any solution protocol must
be easily extensible to handle other requirements as they are
introduced in application-specific requirements documents. Examples
of such application-specific requirements are [RFC4927], [RFC5376],
and [INTER-LAYER].
This document specifies the Path Computation Element Protocol (PCEP)
for communications between a PCC and a PCE, or between two PCEs, in
compliance with [RFC4657]. Such interactions include path
computation requests and path computation replies as well as
notifications of specific states related to the use of a PCE in the
context of MPLS and GMPLS Traffic Engineering.
PCEP is designed to be flexible and extensible so as to easily allow
for the addition of further messages and objects, should further
requirements be expressed in the future.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Terminology
The following terminology is used in this document.
AS: Autonomous System.
Explicit path: Full explicit path from start to destination; made of
a list of strict hops where a hop may be an abstract node such as
an AS.
IGP area: OSPF area or IS-IS level.
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RFC 5440 PCEP March 2009
Inter-domain TE LSP: A TE LSP whose path transits at least two
different domains where a domain can be an IGP area, an Autonomous
System, or a sub-AS (BGP confederation).
PCC: Path Computation Client; any client application requesting a
path computation to be performed by a Path Computation Element.
PCE: Path Computation Element; an entity (component, application, or
network node) that is capable of computing a network path or route
based on a network graph and applying computational constraints.
PCEP Peer: An element involved in a PCEP session (i.e., a PCC or a
PCE).
TED: Traffic Engineering Database that contains the topology and
resource information of the domain. The TED may be fed by IGP
extensions or potentially by other means.
TE LSP: Traffic Engineering Label Switched Path.
Strict/loose path: A mix of strict and loose hops comprising at
least one loose hop representing the destination where a hop may
be an abstract node such as an AS.
Within this document, when describing PCE-PCE communications, the
requesting PCE fills the role of a PCC. This provides a saving in
documentation without loss of function.
The message formats in this document are specified using Backus-Naur
Format (BNF) encoding as specified in [RBNF].
3. Assumptions
[RFC4655] describes various types of PCE. PCEP does not make any
assumption about, and thus does not impose any constraint on, the
nature of the PCE.
Moreover, it is assumed that the PCE has the required information
(usually including network topology and resource information) so as
to perform the computation of a path for a TE LSP. Such information
can be gathered by routing protocols or by some other means. The way
in which the information is gathered is out of the scope of this
document.
Similarly, no assumption is made about the discovery method used by a
PCC to discover a set of PCEs (e.g., via static configuration or
dynamic discovery) and on the algorithm used to select a PCE. For
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RFC 5440 PCEP March 2009
reference, [RFC4674] defines a list of requirements for dynamic PCE
discovery and IGP-based solutions for such PCE discovery are
specified in [RFC5088] and [RFC5089].
4. Architectural Protocol Overview (Model)
The aim of this section is to describe the PCEP model in the spirit
of [RFC4101]. An architectural protocol overview (the big picture of
the protocol) is provided in this section. Protocol details can be
found in further sections.
4.1. Problem
The PCE-based architecture used for the computation of paths for MPLS
and GMPLS TE LSPs is described in [RFC4655]. When the PCC and the
PCE are not collocated, a communication protocol between the PCC and
the PCE is needed. PCEP is such a protocol designed specifically for
communications between a PCC and a PCE or between two PCEs in
compliance with [RFC4657]: a PCC may use PCEP to send a path
computation request for one or more TE LSPs to a PCE, and the PCE may
reply with a set of computed paths if one or more paths can be found
that satisfies the set of constraints.
4.2. Architectural Protocol Overview
PCEP operates over TCP, which fulfills the requirements for reliable
messaging and flow control without further protocol work.
Several PCEP messages are defined:
o Open and Keepalive messages are used to initiate and maintain a
PCEP session, respectively.
o PCReq: a PCEP message sent by a PCC to a PCE to request a path
computation.
o PCRep: a PCEP message sent by a PCE to a PCC in reply to a path
computation request. A PCRep message can contain either a set of
computed paths if the request can be satisfied, or a negative
reply if not. The negative reply may indicate the reason why no
path could be found.
o PCNtf: a PCEP notification message either sent by a PCC to a PCE
or sent by a PCE to a PCC to notify of a specific event.
o PCErr: a PCEP message sent upon the occurrence of a protocol error
condition.
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RFC 5440 PCEP March 2009
o Close message: a message used to close a PCEP session.
The set of available PCEs may be either statically configured on a
PCC or dynamically discovered. The mechanisms used to discover one
or more PCEs and to select a PCE are out of the scope of this
document.
A PCC may have PCEP sessions with more than one PCE, and similarly a
PCE may have PCEP sessions with multiple PCCs.
Each PCEP message is regarded as a single transmission unit and parts
of messages MUST NOT be interleaved. So, for example, a PCC sending
a PCReq and wishing to close the session, must complete sending the
request message before starting to send a Close message.
4.2.1. Initialization Phase
The initialization phase consists of two successive steps (described
in a schematic form in Figure 1):
1) Establishment of a TCP connection (3-way handshake) between the
PCC and the PCE.
2) Establishment of a PCEP session over the TCP connection.
Once the TCP connection is established, the PCC and the PCE (also
referred to as "PCEP peers") initiate PCEP session establishment
during which various session parameters are negotiated. These
parameters are carried within Open messages and include the Keepalive
timer, the DeadTimer, and potentially other detailed capabilities and
policy rules that specify the conditions under which path computation
requests may be sent to the PCE. If the PCEP session establishment
phase fails because the PCEP peers disagree on the session parameters
or one of the PCEP peers does not answer after the expiration of the
establishment timer, the TCP connection is immediately closed.
Successive retries are permitted but an implementation should make
use of an exponential back-off session establishment retry procedure.
Keepalive messages are used to acknowledge Open messages, and are
used once the PCEP session has been successfully established.
Only one PCEP session can exist between a pair of PCEP peers at any
one time. Only one TCP connection on the PCEP port can exist between
a pair of PCEP peers at any one time.
Details about the Open message and the Keepalive message can be found
in Sections 6.2 and 6.3, respectively.
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RFC 5440 PCEP March 2009
+-+-+ +-+-+
|PCC| |PCE|
+-+-+ +-+-+
| |
| Open msg |
|-------- |
| \ Open msg |
| \ ---------|
| \/ |
| /\ |
| / -------->|
| / |
||
| |
Figure 1: PCEP Initialization Phase (Initiated by a PCC)
(Note that once the PCEP session is established, the exchange of
Keepalive messages is optional.)
4.2.2. Session Keepalive
Once a session has been established, a PCE or PCC may want to know
that its PCEP peer is still available for use.
It can rely on TCP for this information, but it is possible that the
remote PCEP function has failed without disturbing the TCP
connection. It is also possible to rely on the mechanisms built into
the TCP implementations, but these might not provide failure
notifications that are sufficiently timely. Lastly, a PCC could wait
until it has a path computation request to send and could use its
failed transmission or the failure to receive a response as evidence
that the session has failed, but this is clearly inefficient.
In order to handle this situation, PCEP includes a keepalive
mechanism based on a Keepalive timer, a DeadTimer, and a Keepalive
message.
Each end of a PCEP session runs a Keepalive timer. It restarts the
timer every time it sends a message on the session. When the timer
expires, it sends a Keepalive message. Other traffic may serve as
Keepalive (see Section 6.3).
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RFC 5440 PCEP March 2009
The ends of the PCEP session also run DeadTimers, and they restart
the timers whenever a message is received on the session. If one end
of the session receives no message before the DeadTimer expires, it
declares the session dead.
Note that this means that the Keepalive message is unresponded and
does not form part of a two-way keepalive handshake as used in some
protocols. Also note that the mechanism is designed to reduce to a
minimum the amount of keepalive traffic on the session.
The keepalive traffic on the session may be unbalanced according to
the requirements of the session ends. Each end of the session can
specify (on an Open message) the Keepalive timer that it will use
(i.e., how often it will transmit a Keepalive message if there is no
other traffic) and a DeadTimer that it recommends its peer to use
(i.e., how long the peer should wait before declaring the session
dead if it receives no traffic). The session ends may use different
Keepalive timer values.
The minimum value of the Keepalive timer is 1 second, and it is
specified in units of 1 second. The recommended default value is 30
seconds. The timer may be disabled by setting it to zero.
The recommended default for the DeadTimer is 4 times the value of the
Keepalive timer used by the remote peer. This means that there is
never any risk of congesting TCP with excessive Keepalive messages.
4.2.3. Path Computation Request Sent by a PCC to a PCE
+-+-+ +-+-+
|PCC| |PCE|
+-+-+ +-+-+
1) Path computation | |
event | |
2) PCE Selection | |
3) Path computation |---- PCReq message--->|
request sent to | |
the selected PCE | |
Figure 2: Path Computation Request
Once a PCC has successfully established a PCEP session with one or
more PCEs, if an event is triggered that requires the computation of
a set of paths, the PCC first selects one or more PCEs. Note that
the PCE selection decision process may have taken place prior to the
PCEP session establishment.
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RFC 5440 PCEP March 2009
Once the PCC has selected a PCE, it sends a path computation request
to the PCE (PCReq message) that contains a variety of objects that
specify the set of constraints and attributes for the path to be
computed. For example, "Compute a TE LSP path with source IP
address=x.y.z.t, destination IP address=x'.y'.z'.t', bandwidth=B
Mbit/s, Setup/Holding priority=P, ...". Additionally, the PCC may
desire to specify the urgency of such request by assigning a request
priority. Each request is uniquely identified by a request-id number
and the PCC-PCE address pair. The process is shown in a schematic
form in Figure 2.
Note that multiple path computation requests may be outstanding from
a PCC to a PCE at any time.
Details about the PCReq message can be found in Section 6.4.
4.2.4. Path Computation Reply Sent by The PCE to a PCC
+-+-+ +-+-+
|PCC| |PCE|
+-+-+ +-+-+
| |
|---- PCReq message--->|
| |1) Path computation
| | request received
| |
| |2) Path successfully
| | computed
| |
| |3) Computed paths
| | sent to the PCC
| |
|
RFC 5440 PCEP March 2009
+-+-+ +-+-+
|PCC| |PCE|
+-+-+ +-+-+
| |
| |
|---- PCReq message--->|
| |1) Path computation
| | request received
| |
| |2) No Path found that
| | satisfies the request
| |
| |3) Negative reply sent to
| | the PCC (optionally with
| | various additional
| | information)
|
RFC 5440 PCEP March 2009
the PCC may decide to redirect its path computation requests to
another PCE should an alternate PCE be available. Similarly, a PCC
may desire to notify a PCE of a particular event such as the
cancellation of pending requests.
+-+-+ +-+-+
|PCC| |PCE|
+-+-+ +-+-+
1) Path computation | |
event | |
2) PCE Selection | |
3) Path computation |---- PCReq message--->|
request X sent to | |4) Path computation
the selected PCE | | request queued
| |
| |
5) Path computation | |
request X cancelled| |
|---- PCNtf message -->|
| |6) Path computation
| | request X cancelled
Figure 4: Example of PCC Notification (Cancellation Notification)
Sent to a PCE
+-+-+ +-+-+
|PCC| |PCE|
+-+-+ +-+-+
1) Path computation | |
event | |
2) PCE Selection | |
3) Path computation |---- PCReq message--->|
request X sent to | |4) Path computation
the selected PCE | | request queued
| |
| |
| |5) PCE gets overloaded
| |
| |
| |6) Path computation
| | request X cancelled
| |
|
RFC 5440 PCEP March 2009
4.2.6. Error
The PCEP Error message (also referred to as a PCErr message) is sent
in several situations: when a protocol error condition is met or when
the request is not compliant with the PCEP specification (e.g.,
capability not supported, reception of a message with a mandatory
missing object, policy violation, unexpected message, unknown request
reference).
+-+-+ +-+-+
|PCC| |PCE|
+-+-+ +-+-+
1) Path computation | |
event | |
2) PCE Selection | |
3) Path computation |---- PCReq message--->|
request X sent to | |4) Reception of a
the selected PCE | | malformed object
| |
| |5) Request discarded
| |
|
RFC 5440 PCEP March 2009
4.2.8. Intermittent versus Permanent PCEP Session
An implementation may decide to keep the PCEP session alive (and thus
the corresponding TCP connection) for an unlimited time. (For
instance, this may be appropriate when path computation requests are
sent on a frequent basis so as to avoid opening a TCP connection each
time a path computation request is needed, which would incur
additional processing delays.) Conversely, in some other
circumstances, it may be desirable to systematically open and close a
PCEP session for each PCEP request (for instance, when sending a path
computation request is a rare event).
5. Transport Protocol
PCEP operates over TCP using a registered TCP port (4189). This
allows the requirements of reliable messaging and flow control to be
met without further protocol work. All PCEP messages MUST be sent
using the registered TCP port for the source and destination TCP
port.
6. PCEP Messages
A PCEP message consists of a common header followed by a variable-
length body made of a set of objects that can either be mandatory or
optional. In the context of this document, an object is said to be
mandatory in a PCEP message when the object MUST be included for the
message to be considered valid. A PCEP message with a missing
mandatory object MUST trigger an Error message (see Section 7.15).
Conversely, if an object is optional, the object may or may not be
present.
A flag referred to as the P flag is defined in the common header of
each PCEP object (see Section 7.2). When this flag is set in an
object in a PCReq, the PCE MUST take the information carried in the
object into account during the path computation. For example, the
METRIC object defined in Section 7.8 allows a PCC to specify a
bounded acceptable path cost. The METRIC object is optional, but a
PCC may set a flag to ensure that the constraint is taken into
account. In this case, if the constraint cannot be taken into
account by the PCE, the PCE MUST trigger an Error message.
For each PCEP message type, rules are defined that specify the set of
objects that the message can carry. We use the Backus-Naur Form
(BNF) (see [RBNF]) to specify such rules. Square brackets refer to
optional sub-sequences. An implementation MUST form the PCEP
messages using the object ordering specified in this document.
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RFC 5440 PCEP March 2009
6.1. Common Header
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ver | Flags | Message-Type | Message-Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: PCEP Message Common Header
Ver (Version - 3 bits): PCEP version number. Current version is
version 1.
Flags (5 bits): No flags are currently defined. Unassigned bits are
considered as reserved. They MUST be set to zero on transmission
and MUST be ignored on receipt.
Message-Type (8 bits): The following message types are currently
defined:
Value Meaning
1 Open
2 Keepalive
3 Path Computation Request
4 Path Computation Reply
5 Notification
6 Error
7 Close
Message-Length (16 bits): total length of the PCEP message including
the common header, expressed in bytes.
6.2. Open Message
The Open message is a PCEP message sent by a PCC to a PCE and by a
PCE to a PCC in order to establish a PCEP session. The Message-Type
field of the PCEP common header for the Open message is set to 1.
Once the TCP connection has been successfully established, the first
message sent by the PCC to the PCE or by the PCE to the PCC MUST be
an Open message as specified in Appendix A.
Any message received prior to an Open message MUST trigger a protocol
error condition causing a PCErr message to be sent with Error-Type
"PCEP session establishment failure" and Error-value "reception of an
invalid Open message or a non Open message" and the PCEP session
establishment attempt MUST be terminated by closing the TCP
connection.
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RFC 5440 PCEP March 2009
The Open message is used to establish a PCEP session between the PCEP
peers. During the establishment phase, the PCEP peers exchange
several session characteristics. If both parties agree on such
characteristics, the PCEP session is successfully established.
The format of an Open message is as follows:
::=
The Open message MUST contain exactly one OPEN object (see
Section 7.3).
Various session characteristics are specified within the OPEN object.
Once the TCP connection has been successfully established, the sender
MUST start an initialization timer called OpenWait after the
expiration of which, if no Open message has been received, it sends a
PCErr message and releases the TCP connection (see Appendix A for
details).
Once an Open message has been sent to a PCEP peer, the sender MUST
start an initialization timer called KeepWait after the expiration of
which, if neither a Keepalive message has been received nor a PCErr
message in case of disagreement of the session characteristics, a
PCErr message MUST be sent and the TCP connection MUST be released
(see Appendix A for details).
The OpenWait and KeepWait timers have a fixed value of 1 minute.
Upon the receipt of an Open message, the receiving PCEP peer MUST
determine whether the suggested PCEP session characteristics are
acceptable. If at least one of the characteristics is not acceptable
to the receiving peer, it MUST send an Error message. The Error
message SHOULD also contain the related OPEN object and, for each
unacceptable session parameter, an acceptable parameter value SHOULD
be proposed in the appropriate field of the OPEN object in place of
the originally proposed value. The PCEP peer MAY decide to resend an
Open message with different session characteristics. If a second
Open message is received with the same set of parameters or with
parameters that are still unacceptable, the receiving peer MUST send
an Error message and it MUST immediately close the TCP connection.
Details about error messages can be found in Section 7.15.
Successive retries are permitted, but an implementation SHOULD make
use of an exponential back-off session establishment retry procedure.
If the PCEP session characteristics are acceptable, the receiving
PCEP peer MUST send a Keepalive message (defined in Section 6.3) that
serves as an acknowledgment.
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RFC 5440 PCEP March 2009
The PCEP session is considered as established once both PCEP peers
have received a Keepalive message from their peer.
6.3. Keepalive Message
A Keepalive message is a PCEP message sent by a PCC or a PCE in order
to keep the session in active state. The Keepalive message is also
used in response to an Open message to acknowledge that an Open
message has been received and that the PCEP session characteristics
are acceptable. The Message-Type field of the PCEP common header for
the Keepalive message is set to 2. The Keepalive message does not
contain any object.
PCEP has its own keepalive mechanism used to ensure the liveness of
the PCEP session. This requires the determination of the frequency
at which each PCEP peer sends Keepalive messages. Asymmetric values
may be chosen; thus, there is no constraint mandating the use of
identical keepalive frequencies by both PCEP peers. The DeadTimer is
defined as the period of time after the expiration of which a PCEP
peer declares the session down if no PCEP message has been received
(Keepalive or any other PCEP message); thus, any PCEP message acts as
a Keepalive message. Similarly, there are no constraints mandating
the use of identical DeadTimers by both PCEP peers. The minimum
Keepalive timer value is 1 second. Deployments SHOULD consider
carefully the impact of using low values for the Keepalive timer as
these might not give rise to the expected results in periods of
temporary network instability.
Keepalive messages are sent at the frequency specified in the OPEN
object carried within an Open message according to the rules
specified in Section 7.3. Because any PCEP message may serve as
Keepalive, an implementation may either decide to send Keepalive
messages at fixed intervals regardless of whether other PCEP messages
might have been sent since the last sent Keepalive message, or may
decide to differ the sending of the next Keepalive message based on
the time at which the last PCEP message (other than Keepalive) was
sent.
Note that sending Keepalive messages to keep the session alive is
optional, and PCEP peers may decide not to send Keepalive messages
once the PCEP session is established; in which case, the peer that
does not receive Keepalive messages does not expect to receive them
and MUST NOT declare the session as inactive.
The format of a Keepalive message is as follows:
::=
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6.4. Path Computation Request (PCReq) Message
A Path Computation Request message (also referred to as a PCReq
message) is a PCEP message sent by a PCC to a PCE to request a path
computation. A PCReq message may carry more than one path
computation request. The Message-Type field of the PCEP common
header for the PCReq message is set to 3.
There are two mandatory objects that MUST be included within a PCReq
message: the RP and the END-POINTS objects (see Section 7). If one
or both of these objects is missing, the receiving PCE MUST send an
error message to the requesting PCC. Other objects are optional.
The format of a PCReq message is as follows:
::=
[]
where:
::=[]
::=[]
::=
[]
[]
[]
[[]]
[]
[]
where:
::=[]
The SVEC, RP, END-POINTS, LSPA, BANDWIDTH, METRIC, RRO, IRO, and
LOAD-BALANCING objects are defined in Section 7. The special case of
two BANDWIDTH objects is discussed in detail in Section 7.7.
A PCEP implementation is free to process received requests in any
order. For example, the requests may be processed in the order they
are received, reordered and assigned priority according to local
policy, reordered according to the priority encoded in the RP object
(Section 7.4.1), or processed in parallel.
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6.5. Path Computation Reply (PCRep) Message
The PCEP Path Computation Reply message (also referred to as a PCRep
message) is a PCEP message sent by a PCE to a requesting PCC in
response to a previously received PCReq message. The Message-Type
field of the PCEP common header for the PCRep message is set to 4.
The bundling of multiple replies to a set of path computation
requests within a single PCRep message is supported by PCEP. If a
PCE receives non-synchronized path computation requests by means of
one or more PCReq messages from a requesting PCC, it MAY decide to
bundle the computed paths within a single PCRep message so as to
reduce the control plane load. Note that the counter side of such an
approach is the introduction of additional delays for some path
computation requests of the set. Conversely, a PCE that receives
multiple requests within the same PCReq message MAY decide to provide
each computed path in separate PCRep messages or within the same
PCRep message. A PCRep message may contain positive and negative
replies.
A PCRep message may contain a set of computed paths corresponding to
either a single path computation request with load-balancing (see
Section 7.16) or multiple path computation requests originated by a
requesting PCC. The PCRep message may also contain multiple
acceptable paths corresponding to the same request.
The PCRep message MUST contain at least one RP object. For each
reply that is bundled into a single PCReq message, an RP object MUST
be included that contains a Request-ID-number identical to the one
specified in the RP object carried in the corresponding PCReq message
(see Section 7.4 for the definition of the RP object).
If the path computation request can be satisfied (i.e., the PCE finds
a set of paths that satisfy the set of constraints), the set of
computed paths specified by means of Explicit Route Objects (EROs) is
inserted in the PCRep message. The ERO is defined in Section 7.9.
The situation where multiple computed paths are provided in a PCRep
message is discussed in detail in Section 7.13. Furthermore, when a
PCC requests the computation of a set of paths for a total amount of
bandwidth by means of a LOAD-BALANCING object carried within a PCReq
message, the ERO of each computed path may be followed by a BANDWIDTH
object as discussed in section Section 7.16.
If the path computation request cannot be satisfied, the PCRep
message MUST include a NO-PATH object. The NO-PATH object (described
in Section 7.5) may also contain other information (e.g, reasons for
the path computation failure).
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The format of a PCRep message is as follows:
::=
where:
::=[]
::=
[]
[]
[]
::=[]
::=
where:
::=[]
[]
[]
[]
::=[]
6.6. Notification (PCNtf) Message
The PCEP Notification message (also referred to as the PCNtf message)
can be sent either by a PCE to a PCC, or by a PCC to a PCE, to notify
of a specific event. The Message-Type field of the PCEP common
header for the PCNtf message is set to 5.
The PCNtf message MUST carry at least one NOTIFICATION object and MAY
contain several NOTIFICATION objects should the PCE or the PCC intend
to notify of multiple events. The NOTIFICATION object is defined in
Section 7.14. The PCNtf message MAY also contain RP objects (see
Section 7.4) when the notification refers to particular path
computation requests.
The PCNtf message may be sent by a PCC or a PCE in response to a
request or in an unsolicited manner.
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The format of a PCNtf message is as follows:
::=::= []
::= []
::=[]
::=[]
6.7. Error (PCErr) Message
The PCEP Error message (also referred to as a PCErr message) is sent
in several situations: when a protocol error condition is met or when
the request is not compliant with the PCEP specification (e.g.,
reception of a malformed message, reception of a message with a
mandatory missing object, policy violation, unexpected message,
unknown request reference). The Message-Type field of the PCEP
common header for the PCErr message is set to 6.
The PCErr message is sent by a PCC or a PCE in response to a request
or in an unsolicited manner. If the PCErr message is sent in
response to a request, the PCErr message MUST include the set of RP
objects related to the pending path computation requests that
triggered the error condition. In the latter case (unsolicited), no
RP object is inserted in the PCErr message. For example, no RP
object is inserted in a PCErr when the error condition occurred
during the initialization phase. A PCErr message MUST contain a
PCEP-ERROR object specifying the PCEP error condition. The PCEP-
ERROR object is defined in Section 7.15.
The format of a PCErr message is as follows:
::=
( [] ) |
[]
::=[]
::=[]
::=[]
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RFC 5440 PCEP March 2009
::=[]
The procedure upon the receipt of a PCErr message is defined in
Section 7.15.
6.8. Close Message
The Close message is a PCEP message that is either sent by a PCC to a
PCE or by a PCE to a PCC in order to close an established PCEP
session. The Message-Type field of the PCEP common header for the
Close message is set to 7.
The format of a Close message is as follows:
::=
The Close message MUST contain exactly one CLOSE object (see
Section 6.8). If more than one CLOSE object is present, the first
MUST be processed and subsequent objects ignored.
Upon the receipt of a valid Close message, the receiving PCEP peer
MUST cancel all pending requests, it MUST close the TCP connection
and MUST NOT send any further PCEP messages on the PCEP session.
6.9. Reception of Unknown Messages
A PCEP implementation that receives an unrecognized PCEP message MUST
send a PCErr message with Error-value=2 (capability not supported).
If a PCC/PCE receives unrecognized messages at a rate equal or
greater than MAX-UNKNOWN-MESSAGES unknown message requests per
minute, the PCC/PCE MUST send a PCEP CLOSE message with close
value="Reception of an unacceptable number of unknown PCEP message".
A RECOMMENDED value for MAX-UNKNOWN-MESSAGES is 5. The PCC/PCE MUST
close the TCP session and MUST NOT send any further PCEP messages on
the PCEP session.
7. Object Formats
PCEP objects have a common format. They begin with a common object
header (see Section 7.2). This is followed by object-specific fields
defined for each different object. The object may also include one
or more type-length-value (TLV) encoded data sets. Each TLV has the
same structure as described in Section 7.1.
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7.1. PCEP TLV Format
A PCEP object may include a set of one or more optional TLVs.
All PCEP TLVs have the following format:
Type: 2 bytes
Length: 2 bytes
Value: variable
A PCEP object TLV is comprised of 2 bytes for the type, 2 bytes
specifying the TLV length, and a value field.
The Length field defines the length of the value portion in bytes.
The TLV is padded to 4-bytes alignment; padding is not included in
the Length field (so a 3-byte value would have a length of 3, but the
total size of the TLV would be 8 bytes).
Unrecognized TLVs MUST be ignored.
IANA management of the PCEP Object TLV type identifier codespace is
described in Section 9.
7.2. Common Object Header
A PCEP object carried within a PCEP message consists of one or more
32-bit words with a common header that has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Object-Class | OT |Res|P|I| Object Length (bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// (Object body) //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: PCEP Common Object Header
Object-Class (8 bits): identifies the PCEP object class.
OT (Object-Type - 4 bits): identifies the PCEP object type.
The Object-Class and Object-Type fields are managed by IANA.
The Object-Class and Object-Type fields uniquely identify each
PCEP object.
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Res flags (2 bits): Reserved field. This field MUST be set to zero
on transmission and MUST be ignored on receipt.
P flag (Processing-Rule - 1-bit): the P flag allows a PCC to specify
in a PCReq message sent to a PCE whether the object must be taken
into account by the PCE during path computation or is just
optional. When the P flag is set, the object MUST be taken into
account by the PCE. Conversely, when the P flag is cleared, the
object is optional and the PCE is free to ignore it.
I flag (Ignore - 1 bit): the I flag is used by a PCE in a PCRep
message to indicate to a PCC whether or not an optional object was
processed. The PCE MAY include the ignored optional object in its
reply and set the I flag to indicate that the optional object was
ignored during path computation. When the I flag is cleared, the
PCE indicates that the optional object was processed during the
path computation. The setting of the I flag for optional objects
is purely indicative and optional. The I flag has no meaning in a
PCRep message when the P flag has been set in the corresponding
PCReq message.
If the PCE does not understand an object with the P flag set or
understands the object but decides to ignore the object, the entire
PCEP message MUST be rejected and the PCE MUST send a PCErr message
with Error-Type="Unknown Object" or "Not supported Object" along with
the corresponding RP object. Note that if a PCReq includes multiple
requests, only requests for which an object with the P flag set is
unknown/unrecognized MUST be rejected.
Object Length (16 bits): Specifies the total object length including
the header, in bytes. The Object Length field MUST always be a
multiple of 4, and at least 4. The maximum object content length
is 65528 bytes.
7.3. OPEN Object
The OPEN object MUST be present in each Open message and MAY be
present in a PCErr message. There MUST be only one OPEN object per
Open or PCErr message.
The OPEN object contains a set of fields used to specify the PCEP
version, Keepalive frequency, DeadTimer, and PCEP session ID, along
with various flags. The OPEN object may also contain a set of TLVs
used to convey various session characteristics such as the detailed
PCE capabilities, policy rules, and so on. No TLVs are currently
defined.
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OPEN Object-Class is 1.
OPEN Object-Type is 1.
The format of the OPEN object body is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ver | Flags | Keepalive | DeadTimer | SID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Optional TLVs //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: OPEN Object Format
Ver (3 bits): PCEP version. Current version is 1.
Flags (5 bits): No flags are currently defined. Unassigned bits are
considered as reserved. They MUST be set to zero on transmission
and MUST be ignored on receipt.
Keepalive (8 bits): maximum period of time (in seconds) between two
consecutive PCEP messages sent by the sender of this message. The
minimum value for the Keepalive is 1 second. When set to 0, once
the session is established, no further Keepalive messages are sent
to the remote peer. A RECOMMENDED value for the keepalive
frequency is 30 seconds.
DeadTimer (8 bits): specifies the amount of time after the
expiration of which the PCEP peer can declare the session with the
sender of the Open message to be down if no PCEP message has been
received. The DeadTimer SHOULD be set to 0 and MUST be ignored if
the Keepalive is set to 0. A RECOMMENDED value for the DeadTimer
is 4 times the value of the Keepalive.
Example:
A sends an Open message to B with Keepalive=10 seconds and
DeadTimer=40 seconds. This means that A sends Keepalive messages (or
any other PCEP message) to B every 10 seconds and B can declare the
PCEP session with A down if no PCEP message has been received from A
within any 40-second period.
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SID (PCEP session ID - 8 bits): unsigned PCEP session number that
identifies the current session. The SID MUST be incremented each
time a new PCEP session is established. It is used for logging
and troubleshooting purposes. Each increment SHOULD have a value
of 1 and may cause a wrap back to zero.
The SID is used to disambiguate instances of sessions to the same
peer. A PCEP implementation could use a single source of SIDs
across all peers, or one source for each peer. The former might
constrain the implementation to only 256 concurrent sessions. The
latter potentially requires more states. There is one SID number
in each direction.
Optional TLVs may be included within the OPEN object body to specify
PCC or PCE characteristics. The specification of such TLVs is
outside the scope of this document.
When present in an Open message, the OPEN object specifies the
proposed PCEP session characteristics. Upon receiving unacceptable
PCEP session characteristics during the PCEP session initialization
phase, the receiving PCEP peer (PCE) MAY include an OPEN object
within the PCErr message so as to propose alternative acceptable
session characteristic values.
7.4. RP Object
The RP (Request Parameters) object MUST be carried within each PCReq
and PCRep messages and MAY be carried within PCNtf and PCErr
messages. The RP object is used to specify various characteristics
of the path computation request.
The P flag of the RP object MUST be set in PCReq and PCRep messages
and MUST be cleared in PCNtf and PCErr messages. If the RP object is
received with the P flag set incorrectly according to the rules
stated above, the receiving peer MUST send a PCErr message with
Error-Type=10 and Error-value=1. The corresponding path computation
request MUST be cancelled by the PCE without further notification.
7.4.1. Object Definition
RP Object-Class is 2.
RP Object-Type is 1.
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The format of the RP object body is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |O|B|R| Pri |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request-ID-number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Optional TLVs //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: RP Object Body Format
The RP object body has a variable length and may contain additional
TLVs. No TLVs are currently defined.
Flags (32 bits)
The following flags are currently defined:
o Pri (Priority - 3 bits): the Priority field may be used by the
requesting PCC to specify to the PCE the request's priority from 1
to 7. The decision of which priority should be used for a
specific request is a local matter; it MUST be set to 0 when
unused. Furthermore, the use of the path computation request
priority by the PCE's scheduler is implementation specific and out
of the scope of this document. Note that it is not required for a
PCE to support the priority field: in this case, it is RECOMMENDED
that the PCC set the priority field to 0 in the RP object. If the
PCE does not take into account the request priority, it is
RECOMMENDED to set the priority field to 0 in the RP object
carried within the corresponding PCRep message, regardless of the
priority value contained in the RP object carried within the
corresponding PCReq message. A higher numerical value of the
priority field reflects a higher priority. Note that it is the
responsibility of the network administrator to make use of the
priority values in a consistent manner across the various PCCs.
The ability of a PCE to support request prioritization MAY be
dynamically discovered by the PCCs by means of PCE capability
discovery. If not advertised by the PCE, a PCC may decide to set
the request priority and will learn the ability of the PCE to
support request prioritization by observing the Priority field of
the RP object received in the PCRep message. If the value of the
Pri field is set to 0, this means that the PCE does not support
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the handling of request priorities: in other words, the path
computation request has been honored but without taking the
request priority into account.
o R (Reoptimization - 1 bit): when set, the requesting PCC specifies
that the PCReq message relates to the reoptimization of an
existing TE LSP. For all TE LSPs except zero-bandwidth LSPs, when
the R bit is set, an RRO (see Section 7.10) MUST be included in
the PCReq message to show the path of the existing TE LSP. Also,
for all TE LSPs except zero-bandwidth LSPs, when the R bit is set,
the existing bandwidth of the TE LSP to be reoptimized MUST be
supplied in a BANDWIDTH object (see Section 7.7). This BANDWIDTH
object is in addition to the instance of that object used to
describe the desired bandwidth of the reoptimized LSP. For zero-
bandwidth LSPs, the RRO and BANDWIDTH objects that report the
characteristics of the existing TE LSP are optional.
o B (Bi-directional - 1 bit): when set, the PCC specifies that the
path computation request relates to a bi-directional TE LSP that
has the same traffic engineering requirements including fate
sharing, protection and restoration, LSRs, TE links, and resource
requirements (e.g., latency and jitter) in each direction. When
cleared, the TE LSP is unidirectional.
o O (strict/loose - 1 bit): when set, in a PCReq message, this
indicates that a loose path is acceptable. Otherwise, when
cleared, this indicates to the PCE that a path exclusively made of
strict hops is required. In a PCRep message, when the O bit is
set this indicates that the returned path is a loose path;
otherwise (when the O bit is cleared), the returned path is made
of strict hops.
Unassigned bits are considered reserved. They MUST be set to zero on
transmission and MUST be ignored on receipt.
Request-ID-number (32 bits): The Request-ID-number value combined
with the source IP address of the PCC and the PCE address uniquely
identify the path computation request context. The Request-ID-
number is used for disambiguation between pending requests, and
thus it MUST be changed (such as by incrementing it) each time a
new request is sent to the PCE, and may wrap.
The value 0x00000000 is considered invalid.
If no path computation reply is received from the PCE (e.g., the
request is dropped by the PCE because of memory overflow), and the
PCC wishes to resend its request, the same Request-ID-number MUST
be used. Upon receiving a path computation request from a PCC
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with the same Request-ID-number, the PCE SHOULD treat the request
as a new request. An implementation MAY choose to cache path
computation replies in order to quickly handle retransmission
without having to process a path computation request twice (in the
case that the first request was dropped or lost). Upon receiving
a path computation reply from a PCE with the same Request-ID-
number, the PCC SHOULD silently discard the path computation
reply.
Conversely, different Request-ID-numbers MUST be used for
different requests sent to a PCE.
The same Request-ID-number MAY be used for path computation
requests sent to different PCEs. The path computation reply is
unambiguously identified by the IP source address of the replying
PCE.
7.4.2. Handling of the RP Object
If a PCReq message is received that does not contain an RP object,
the PCE MUST send a PCErr message to the requesting PCC with Error-
Type="Required Object missing" and Error-value="RP Object missing".
If the O bit of the RP message carried within a PCReq message is
cleared and local policy has been configured on the PCE to not
provide explicit paths (for instance, for confidentiality reasons), a
PCErr message MUST be sent by the PCE to the requesting PCC and the
pending path computation request MUST be discarded. The Error-Type
is "Policy Violation" and Error-value is "O bit cleared".
When the R bit of the RP object is set in a PCReq message, this
indicates that the path computation request relates to the
reoptimization of an existing TE LSP. In this case, the PCC MUST
also provide the strict/loose path by including an RRO object in the
PCReq message so as to avoid/limit double-bandwidth counting if and
only if the TE LSP is a non-zero-bandwidth TE LSP. If the PCC has
not requested a strict path (O bit set), a reoptimization can still
be requested by the PCC, but this requires that the PCE either be
stateful (keep track of the previously computed path with the
associated list of strict hops), or have the ability to retrieve the
complete required path segment. Alternatively, the PCC MUST inform
the PCE about the working path and the associated list of strict hops
in PCReq. The absence of an RRO in the PCReq message for a non-zero-
bandwidth TE LSP (when the R bit of the RP object is set) MUST
trigger the sending of a PCErr message with Error-Type="Required
Object Missing" and Error-value="RRO Object missing for
reoptimization".
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If a PCC/PCE receives a PCRep/PCReq message that contains an RP
object referring to an unknown Request-ID-number, the PCC/PCE MUST
send a PCErr message with Error-Type="Unknown request reference".
This is used for debugging purposes. If a PCC/PCE receives PCRep/
PCReq messages with unknown requests at a rate equal or greater than
MAX-UNKNOWN-REQUESTS unknown requests per minute, the PCC/PCE MUST
send a PCEP CLOSE message with close value="Reception of an
unacceptable number of unknown requests/replies". A RECOMMENDED
value for MAX-UNKNOWN-REQUESTS is 5. The PCC/PCE MUST close the TCP
session and MUST NOT send any further PCEP messages on the PCEP
session.
The reception of a PCEP message that contains an RP object referring
to a Request-ID-number=0x00000000 MUST be treated in similar manner
as an unknown request.
7.5. NO-PATH Object
The NO-PATH object is used in PCRep messages in response to an
unsuccessful path computation request (the PCE could not find a path
satisfying the set of constraints). When a PCE cannot find a path
satisfying a set of constraints, it MUST include a NO-PATH object in
the PCRep message.
There are several categories of issue that can lead to a negative
reply. For example, the PCE chain might be broken (should there be
more than one PCE involved in the path computation) or no path
obeying the set constraints could be found. The "NI (Nature of
Issue)" field in the NO-PATH object is used to report the error
category.
Optionally, if the PCE supports such capability, the NO-PATH object
MAY contain an optional NO-PATH-VECTOR TLV defined below and used to
provide more information on the reasons that led to a negative reply.
The PCRep message MAY also contain a list of objects that specify the
set of constraints that could not be satisfied. The PCE MAY just
replicate the set of objects that was received that was the cause of
the unsuccessful computation or MAY optionally report a suggested
value for which a path could have been found (in which case, the
value differs from the value in the original request).
NO-PATH Object-Class is 3.
NO-PATH Object-Type is 1.
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The format of the NO-PATH object body is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Nature of Issue|C| Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Optional TLVs //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: NO-PATH Object Format
NI - Nature of Issue (8 bits): The NI field is used to report the
nature of the issue that led to a negative reply. Two values are
currently defined:
0: No path satisfying the set of constraints could be found
1: PCE chain broken
The Nature of Issue field value can be used by the PCC for various
purposes:
* Constraint adjustment before reissuing a new path computation
request,
* Explicit selection of a new PCE chain,
* Logging of the error type for further action by the network
administrator.
IANA management of the NI field codespace is described in
Section 9.
Flags (16 bits).
The following flag is currently defined:
o C flag (1 bit): when set, the PCE indicates the set of unsatisfied
constraints (reasons why a path could not be found) in the PCRep
message by including the relevant PCEP objects. When cleared, no
failing constraints are specified. The C flag has no meaning and
is ignored unless the NI field is set to 0x00.
Unassigned bits are considered as reserved. They MUST be set to zero
on transmission and MUST be ignored on receipt.
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Reserved (8 bits): This field MUST be set to zero on transmission
and MUST be ignored on receipt.
The NO-PATH object body has a variable length and may contain
additional TLVs. The only TLV currently defined is the NO-PATH-
VECTOR TLV defined below.
Example: consider the case of a PCC that sends a path computation
request to a PCE for a TE LSP of X Mbit/s. Suppose that PCE cannot
find a path for X Mbit/s. In this case, the PCE must include in the
PCRep message a NO-PATH object. Optionally, the PCE may also include
the original BANDWIDTH object so as to indicate that the reason for
the unsuccessful computation is the bandwidth constraint (in this
case, the NI field value is 0x00 and C flag is set). If the PCE
supports such capability, it may alternatively include the BANDWIDTH
object and report a value of Y in the bandwidth field of the
BANDWIDTH object (in this case, the C flag is set) where Y refers to
the bandwidth for which a TE LSP with the same other characteristics
(such as Setup/Holding priorities, TE LSP attribute, local
protection, etc.) could have been computed.
When the NO-PATH object is absent from a PCRep message, the path
computation request has been fully satisfied and the corresponding
paths are provided in the PCRep message.
An optional TLV named NO-PATH-VECTOR MAY be included in the NO-PATH
object in order to provide more information on the reasons that led
to a negative reply.
The NO-PATH-VECTOR TLV is compliant with the PCEP TLV format defined
in Section 7.1 and is comprised of 2 bytes for the type, 2 bytes
specifying the TLV length (length of the value portion in bytes)
followed by a fixed-length 32-bit flags field.
Type: 1
Length: 4 bytes
Value: 32-bit flags field
IANA manages the space of flags carried in the NO-PATH-VECTOR TLV
(see Section 9).
The following flags are currently defined:
o Bit number: 31 - PCE currently unavailable
o Bit number: 30 - Unknown destination
o Bit number: 29 - Unknown source
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7.6. END-POINTS Object
The END-POINTS object is used in a PCReq message to specify the
source IP address and the destination IP address of the path for
which a path computation is requested. The P flag of the END-POINTS
object MUST be set. If the END-POINTS object is received with the P
flag cleared, the receiving peer MUST send a PCErr message with
Error-Type=10 and Error-value=1. The corresponding path computation
request MUST be cancelled by the PCE without further notification.
Note that the source and destination addresses specified in the END-
POINTS object may correspond to the source and destination IP address
of the TE LSP or to those of a path segment. Two END-POINTS objects
(for IPv4 and IPv6) are defined.
END-POINTS Object-Class is 4.
END-POINTS Object-Type is 1 for IPv4 and 2 for IPv6.
The format of the END-POINTS object body for IPv4 (Object-Type=1) is
as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IPv4 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination IPv4 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: END-POINTS Object Body Format for IPv4
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The format of the END-POINTS object for IPv6 (Object-Type=2) is as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Source IPv6 address (16 bytes) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Destination IPv6 address (16 bytes) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13: END-POINTS Object Body Format for IPv6
The END-POINTS object body has a fixed length of 8 bytes for IPv4 and
32 bytes for IPv6.
If more than one END-POINTS object is present, the first MUST be
processed and subsequent objects ignored.
7.7. BANDWIDTH Object
The BANDWIDTH object is used to specify the requested bandwidth for a
TE LSP. The notion of bandwidth is similar to the one used for RSVP
signaling in [RFC2205], [RFC3209], and [RFC3473].
If the requested bandwidth is equal to 0, the BANDWIDTH object is
optional. Conversely, if the requested bandwidth is not equal to 0,
the PCReq message MUST contain a BANDWIDTH object.
In the case of the reoptimization of a TE LSP, the bandwidth of the
existing TE LSP MUST also be included in addition to the requested
bandwidth if and only if the two values differ. Consequently, two
Object-Type values are defined that refer to the requested bandwidth
and the bandwidth of the TE LSP for which a reoptimization is being
performed.
The BANDWIDTH object may be carried within PCReq and PCRep messages.
BANDWIDTH Object-Class is 5.
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Two Object-Type values are defined for the BANDWIDTH object:
o Requested bandwidth: BANDWIDTH Object-Type is 1.
o Bandwidth of an existing TE LSP for which a reoptimization is
requested. BANDWIDTH Object-Type is 2.
The format of the BANDWIDTH object body is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: BANDWIDTH Object Body Format
Bandwidth (32 bits): The requested bandwidth is encoded in 32 bits
in IEEE floating point format (see [IEEE.754.1985]), expressed in
bytes per second. Refer to Section 3.1.2 of [RFC3471] for a table
of commonly used values.
The BANDWIDTH object body has a fixed length of 4 bytes.
7.8. METRIC Object
The METRIC object is optional and can be used for several purposes.
In a PCReq message, a PCC MAY insert one or more METRIC objects:
o To indicate the metric that MUST be optimized by the path
computation algorithm (IGP metric, TE metric, hop counts).
Currently, three metrics are defined: the IGP cost, the TE metric
(see [RFC3785]), and the number of hops traversed by a TE LSP.
o To indicate a bound on the path cost that MUST NOT be exceeded for
the path to be considered as acceptable by the PCC.
In a PCRep message, the METRIC object MAY be inserted so as to
provide the cost for the computed path. It MAY also be inserted
within a PCRep with the NO-PATH object to indicate that the metric
constraint could not be satisfied.
The path computation algorithmic aspects used by the PCE to optimize
a path with respect to a specific metric are outside the scope of
this document.
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It must be understood that such path metrics are only meaningful if
used consistently: for instance, if the delay of a computed path
segment is exchanged between two PCEs residing in different domains,
consistent ways of defining the delay must be used.
The absence of the METRIC object MUST be interpreted by the PCE as a
path computation request for which no constraints need be applied to
any of the metrics.
METRIC Object-Class is 6.
METRIC Object-Type is 1.
The format of the METRIC object body is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Flags |C|B| T |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| metric-value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 15: METRIC Object Body Format
The METRIC object body has a fixed length of 8 bytes.
Reserved (16 bits): This field MUST be set to zero on transmission
and MUST be ignored on receipt.
T (Type - 8 bits): Specifies the metric type.
Three values are currently defined:
* T=1: IGP metric
* T=2: TE metric
* T=3: Hop Counts
Flags (8 bits): Two flags are currently defined:
* B (Bound - 1 bit): When set in a PCReq message, the metric-
value indicates a bound (a maximum) for the path metric that
must not be exceeded for the PCC to consider the computed path
as acceptable. The path metric must be less than or equal to
the value specified in the metric-value field. When the B flag
is cleared, the metric-value field is not used to reflect a
bound constraint.
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* C (Computed Metric - 1 bit): When set in a PCReq message, this
indicates that the PCE MUST provide the computed path metric
value (should a path satisfying the constraints be found) in
the PCRep message for the corresponding metric.
Unassigned flags MUST be set to zero on transmission and MUST be
ignored on receipt.
Metric-value (32 bits): metric value encoded in 32 bits in IEEE
floating point format (see [IEEE.754.1985]).
Multiple METRIC objects MAY be inserted in a PCRep or a PCReq message
for a given request (i.e., for a given RP). For a given request,
there MUST be at most one instance of the METRIC object for each
metric type with the same B flag value. If, for a given request, two
or more instances of a METRIC object with the same B flag value are
present for a metric type, only the first instance MUST be considered
and other instances MUST be ignored.
For a given request, the presence of two METRIC objects of the same
type with a different value of the B flag is allowed. Furthermore,
it is also allowed to insert, for a given request, two METRIC objects
with different types that have both their B flag cleared: in this
case, an objective function must be used by the PCE to solve a multi-
parameter optimization problem.
A METRIC object used to indicate the metric to optimize during the
path computation MUST have the B flag cleared and the C flag set to
the appropriate value. When the path computation relates to the
reoptimization of an exiting TE LSP (in which case, the R flag of the
RP object is set), an implementation MAY decide to set the metric-
value field to the computed value of the metric of the TE LSP to be
reoptimized with regards to a specific metric type.
A METRIC object used to reflect a bound MUST have the B flag set, and
the C flag and metric-value field set to the appropriate values.
In a PCRep message, unless not allowed by PCE policy, at least one
METRIC object MUST be present that reports the computed path metric
if the C flag of the METRIC object was set in the corresponding path
computation request (the B flag MUST be cleared). The C flag has no
meaning in a PCRep message. Optionally, the PCRep message MAY
contain additional METRIC objects that correspond to bound
constraints; in which case, the metric-value MUST be equal to the
corresponding computed path metric (the B flag MUST be set). If no
path satisfying the constraints could be found by the PCE, the METRIC
objects MAY also be present in the PCRep message with the NO-PATH
object to indicate the constraint metric that could be satisfied.
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Example: if a PCC sends a path computation request to a PCE where the
metric to optimize is the IGP metric and the TE metric must not
exceed the value of M, two METRIC objects are inserted in the PCReq
message:
o First METRIC object with B=0, T=1, C=1, metric-value=0x0000
o Second METRIC object with B=1, T=2, metric-value=M
If a path satisfying the set of constraints can be found by the PCE
and there is no policy that prevents the return of the computed
metric, the PCE inserts one METRIC object with B=0, T=1, metric-
value= computed IGP path cost. Additionally, the PCE may insert a
second METRIC object with B=1, T=2, metric-value= computed TE path
cost.
7.9. Explicit Route Object
The ERO is used to encode the path of a TE LSP through the network.
The ERO is carried within a PCRep message to provide the computed TE
LSP if the path computation was successful.
The contents of this object are identical in encoding to the contents
of the Resource Reservation Protocol Traffic Engineering Extensions
(RSVP-TE) Explicit Route Object (ERO) defined in [RFC3209],
[RFC3473], and [RFC3477]. That is, the object is constructed from a
series of sub-objects. Any RSVP-TE ERO sub-object already defined or
that could be defined in the future for use in the RSVP-TE ERO is
acceptable in this object.
PCEP ERO sub-object types correspond to RSVP-TE ERO sub-object types.
Since the explicit path is available for immediate signaling by the
MPLS or GMPLS control plane, the meanings of all of the sub-objects
and fields in this object are identical to those defined for the ERO.
ERO Object-Class is 7.
ERO Object-Type is 1.
7.10. Reported Route Object
The RRO is exclusively carried within a PCReq message so as to report
the route followed by a TE LSP for which a reoptimization is desired.
The contents of this object are identical in encoding to the contents
of the Route Record Object defined in [RFC3209], [RFC3473], and
[RFC3477]. That is, the object is constructed from a series of sub-
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objects. Any RSVP-TE RRO sub-object already defined or that could be
defined in the future for use in the RSVP-TE RRO is acceptable in
this object.
The meanings of all of the sub-objects and fields in this object are
identical to those defined for the RSVP-TE RRO.
PCEP RRO sub-object types correspond to RSVP-TE RRO sub-object types.
RRO Object-Class is 8.
RRO Object-Type is 1.
7.11. LSPA Object
The LSPA (LSP Attributes) object is optional and specifies various TE
LSP attributes to be taken into account by the PCE during path
computation. The LSPA object can be carried within a PCReq message,
or a PCRep message in case of unsuccessful path computation (in this
case, the PCRep message also contains a NO-PATH object, and the LSPA
object is used to indicate the set of constraints that could not be
satisfied). Most of the fields of the LSPA object are identical to
the fields of the SESSION-ATTRIBUTE object (C-Type = 7) defined in
[RFC3209] and [RFC4090]. When absent from the PCReq message, this
means that the Setup and Holding priorities are equal to 0, and there
are no affinity constraints. See Section 4.7.4 of [RFC3209] for a
detailed description of the use of resource affinities.
LSPA Object-Class is 9.
LSPA Object-Types is 1.
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The format of the LSPA object body is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Exclude-any |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Include-any |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Include-all |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Setup Prio | Holding Prio | Flags |L| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Optional TLVs //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 16: LSPA Object Body Format
Setup Prio (Setup Priority - 8 bits): The priority of the TE LSP
with respect to taking resources, in the range of 0 to 7. The
value 0 is the highest priority. The Setup Priority is used in
deciding whether this session can preempt another session.
Holding Prio (Holding Priority - 8 bits): The priority of the TE LSP
with respect to holding resources, in the range of 0 to 7. The
value 0 is the highest priority. Holding Priority is used in
deciding whether this session can be preempted by another session.
Flags (8 bits)
L flag: Corresponds to the "Local Protection Desired" bit
([RFC3209]) of the SESSION-ATTRIBUTE Object. When set, this
means that the computed path must include links protected with
Fast Reroute as defined in [RFC4090].
Unassigned flags MUST be set to zero on transmission and MUST be
ignored on receipt.
Reserved (8 bits): This field MUST be set to zero on transmission
and MUST be ignored on receipt.
Note that optional TLVs may be defined in the future to carry
additional TE LSP attributes such as those defined in [RFC5420].
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7.12. Include Route Object
The IRO (Include Route Object) is optional and can be used to specify
that the computed path MUST traverse a set of specified network
elements. The IRO MAY be carried within PCReq and PCRep messages.
When carried within a PCRep message with the NO-PATH object, the IRO
indicates the set of elements that cause the PCE to fail to find a
path.
IRO Object-Class is 10.
IRO Object-Type is 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// (Sub-objects) //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 17: IRO Body Format
Sub-objects: The IRO is made of sub-objects identical to the ones
defined in [RFC3209], [RFC3473], and [RFC3477], where the IRO sub-
object type is identical to the sub-object type defined in the
related documents.
The following sub-object types are supported.
Type Sub-object
1 IPv4 prefix
2 IPv6 prefix
4 Unnumbered Interface ID
32 Autonomous system number
The L bit of such sub-object has no meaning within an IRO.
7.13. SVEC Object
7.13.1. Notion of Dependent and Synchronized Path Computation Requests
Independent versus dependent path computation requests: path
computation requests are said to be independent if they are not
related to each other. Conversely, a set of dependent path
computation requests is such that their computations cannot be
performed independently of each other (a typical example of dependent
requests is the computation of a set of diverse paths).
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Synchronized versus non-synchronized path computation requests: a set
of path computation requests is said to be non-synchronized if their
respective treatment (path computations) can be performed by a PCE in
a serialized and independent fashion.
There are various circumstances where the synchronization of a set of
path computations may be beneficial or required.
Consider the case of a set of N TE LSPs for which a PCC needs to send
path computation requests to a PCE. The first solution consists of
sending N separate PCReq messages to the selected PCE. In this case,
the path computation requests are non-synchronized. Note that the
PCC may chose to distribute the set of N requests across K PCEs for
load balancing purposes. Considering that M (with M